Trivalent chromium based electrolytes have been in use industrially now for many years since the late 1970s. These processes have advantages over those based on hexavalent chromium in terms of health and safety and toxicity to the environment. However, selection of suitable anodes for these trivalent processes can present significant problems. Insoluble anodes have to be used since the cathode efficiency of the process is very low. The low cathode efficiency would cause a build up of chromium metal in the bath if soluble anodes made of chromium were used. Also, chromium is passive in the electrolyte until an anodic potential sufficient to dissolve the chromium as Cr(VI) is reached. This means that chromium would dissolve in a hexavalent rather than trivalent form if chromium metal anodes were used. Hexavalent chromium is a serious contaminant in trivalent processes and it is important to prevent the formation of this species. Historically, there have been several approaches to this problem: Chloride based electrolytes (where chlorine evolution from insoluble anodes may also be a problem) use bromide ions to catalyse anodic oxidation of chemical species such as formate ions or ammonium ions rather than oxidation of chromium(III) to chromium(VI) (for example see U.S. Pat. No. 3,954,574).
Due to the type of additives used in sulfate based trivalent processes, this strategy cannot be used. In sulfate based processes, there are two possible methods of preventing chromium oxidation. Originally, a divided cell arrangement was used with these processes (for example UK Patent No. 1,602,404). Typically, a lead anode was used in a sulfuric acid anolyte which was separated from the plating bath with a permeable membrane. The plating current was carried by hydrogen cations through a cation permeable membrane. This effectively prevented any contact of trivalent chromium with the surface of the anode, thus preventing oxidation of trivalent to hexavalent chromium. However, this type of arrangement was expensive and difficult to maintain. Also, the membrane had a limited lifespan resulting in unfavourable costs. A later development in trivalent chromium electroplating technology from sulfate based electrolytes utilised iridium/tantalum oxide coated anodes (for example see U.S. Pat. No. 5,560,815). These were used directly in the trivalent chromium solution and the surface of these anodes was found to have a low oxygen over potential (thus facilitating oxygen liberation at the lowest possible anode potentials). However, over a period of operation, the oxidation of trivalent to hexavalent chromium on these anodes was facilitated. Because of the problems outlined above, there remains a need for a suitable cost effective anode for sulfate based trivalent chromium plating processes.